Github user cloud-fan commented on a diff in the pull request: https://github.com/apache/spark/pull/10451#discussion_r48432615 --- Diff: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala --- @@ -153,6 +153,15 @@ object SetOperationPushDown extends Rule[LogicalPlan] with PredicateHelper { ) ) + // Push down limit into union + case Limit(exp, Union(left, right)) => + Limit(exp, + Union( + CombineLimits(Limit(exp, left)), + CombineLimits(Limit(exp, right)) + ) + ) --- End diff -- After think it more, there may be a problem: If `left` or `right` is an operator that can push `Limit` down(currently there is no such operator, but we can't guarantee there won't be). Then every time you push down a `Limit` here, it will be pushed down further. Thus the `CombineLimits` can NOT detect that you have already pushed the `Limit` down, and keeps generating new `Limit` and pushing it down. I think we should have a better way to detect whether we have pushed `Limit` down or not, or add some comments to say that this rule assumes the newly added `Limit` on top of `left` and `right` won't be removed by other optimization rules.
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